1. Brief Description of the Invention
The invention relates to a composition that includes a mineral clay compound for use in producing a foundry mold, a foundry mold made of the composition and a method for producing the foundry mold.
2. Description of the Prior Art
In the casting of molten metals and alloys for various foundry applications, it is customary to employ foundry molds having an internal configuration conforming to the shape of the desired casting. Foundry molds for this purpose are conventionally constructed from compositions that include foundry sand as the major constituent in combination with a mineral clay and water. Supplemental additions including ground bituminous coal, lignite, leonardite, pregelatinized starches, cellulose and other conventional additives may also be present in minor amounts. The foundry mold composition is produced conventionally by introducing foundry sand, water, any supplemental additions and the mineral clay into a mixing apparatus, such as a muller, wherein mixing of these constituents is performed to an extent that the particles of the foundry sand are coated by the mineral clay component. It is also common practice to add the supplemental additives and the mineral clay as a single mixture to the foundry sand and water in a mixing apparatus, such as a muller, wherein the mixing of these constituents is performed to an extent that the particles of the foundry sand are coated with the mineral clay. Thereafter, the composition from the muller is introduced to a flask or confining structure incorporating a pattern configured to correspond to the desired configuration of the metal or alloy casting. The composition is consolidated within the pattern to obtain the required integrity, and then the pattern is removed rendering the foundry mold ready for use in producing a metal or alloy casting.
Conventional foundry mold compositions have a mineral or clay compound that includes sodium bentonite clay or calcium bentonite or mixtures of sodium and calcium bentonite to achieve desired properties in the foundry mold composition. Typically, the calcium bentonite constituent would be present in an amount of 0 to 100 percent by weight of the mineral clay mixture. Other mineral clays constituting the well known plastic fine clays, such as kaolinite and illite, may be added to facilitate binding or coating of the foundry sand particles during the mixing operation.
A significant property or characteristic of foundry mold compositions is "durability"; this is the resistance of the mold and specifically the mineral clay component thereof to thermal degradation at the elevated temperatures encountered during conventional hot metal and alloy casting. Specifically, it is advantageous that the mold composition be resistant to thermal degradation at temperatures ranging from 400.degree. to 1200.degree. F. Mineral clays burn-out or lose their desired properties progressively as temperatures increase within this range. It is advantageous to use mineral clays with improved durability to reduce consumption in use. Sodium bentonite exhibits superior durability compared to calcium bentonite in mold compositions. A second significant property is the "dry strength"; this is defined as the energy required to remove the solidified casting from the mold. In conventional foundry mold compositions that contain blends of sodium bentonite and calcium bentonite, the dry strength property of the mold is enhanced by increased amounts of calcium bentonite which serves to reduce dry strength and facilitates easier removal of the casting from the sand mold. This constituent, however, results in degradation of the durability of the mold. A third significant property is moldability of the mold composition. Moldability is defined as the measurement of apparent cohesion between sand grains of the mold composition. Mold compositions deficient in this property will stick in hoppers, to transfer equipment, and can be detrimental to the entire casting process. Mold compositions containing calcium bentonite as all or part of the mineral clay content exhibit improved moldability, particularly when the water content of the mold composition increases. A fourth significant property, "hot strength," is the ability of the mold composition to maintain its integrity at the mold composition/molten metal interface during and following pouring of the metal, usually at temperatures of 1500.degree. F. and above. All of the additional, desired foundry mold properties are attributable to equally by sodium bentonite or calcium bentonite or are enhanced by sodium bentonite in the mold composition. These properties include "green strength" which is the strength or integrity of the mold prior to pouring of the molten metal into the mold. "Wet tensile strength" is the resistance of the mold to degradation due to transient shocks or jolts. "Hot deformation" is the ability of the mold to maintain dimensional stability during hot metal casting so that required dimensional tolerances are achieved with respect to the solidified casting.
Since improved dry strength and improved flowability of the mold composition is enhanced by the presence of calcium bentonite, conventionally, this constituent has been considered to be essential even though it results in degradation of the durability of the mold. Hence, with conventional foundry mold compositions it has not been possible heretofore to achieve an optimum combination of durability and dry strength, along with optimization of the other foundry mold properties discussed above and resulting from increased amounts of sodium bentonite in relation to calcium bentonite.